Sometimes voyeurs use normal cameras, but the photographer is concealed. Sometimes the camera itself is disguised or concealed. Some obvious element of concealment (or great distance) is generally needed to make such photography fall under the category of ‘secret photography’ rather than street photography or documentary photography.

Although spy cameras small enough to fit inside a pocket-watch had existed since the 1880s, advances in miniaturization and electronics since the 1950s has greatly aided the ability to conceal miniature cameras, and the quality and affordability of tiny cameras (often called “spy cameras” or subminiature cameras) has now greatly increased. Some consumer digital cameras are now so small that in previous decades they would have qualified as “spy cameras”, and digital cameras of five megapixels or more are now being embedded in some mobile camera phones.

Some institutions, such as gyms and schools, have banned camera phones because of the privacy issues they raise in areas like changerooms (locker rooms). Saudi Arabia banned the sale of camera phones nationwide for a period, but reversed the ban in 2004. South Korea requires that all camera phones sold in the country make a clearly audible sound whenever a picture is being taken.

Some fine art photographers have displayed a fascination with the forms of secret voyeuristic photography. Voyeuristic photography has also been centrally explored in movies such as Michael Powell’s Peeping Tom, and Michelangelo Antonioni’s Blowup, and has appeared to comic effect in films such as Gregory’s Girl and American Pie.

Secret photography by law enforcement authorities is called surveillance and is not considered to be voyeurism, though it may be unlawful or regulated in some countries.

Certain image capturing devices are capable of producing images through materials which are opaque to visible light, including clothing. These devices form images by using electromagnetic radiation outside the visible range. Infrared and terrahertz-wave cameras are capable of creating images through clothing, though these images differ from what would be created with visible light.

In clinical psychology, voyeurism is the sexual interest in or practice of spying on people engaged in intimate behaviors, such as undressing, sexual activity, or other activity usually considered to be of a private nature. In popular imagination the term is used in a more general sense to refer to someone who habitually observes others without their knowledge, with no necessary implication of sexual interest.

Voyeurism (from the French voyeur, “one who looks”) can take several forms, but its principal characteristic is that the voyeur does not normally relate directly with the subject of their interest, who is often unaware of being observed. The voyeur may observe the subject from a distance, or use stealth to observe the subject with the use of peep-holes, two-way mirrors, hidden cameras, secret photography and other devices and strategies.